The present invention relates to a system for controlling an active suspension of a vehicle.
Active suspension systems of various forms having been developed and disclosed as in Japanese Pat. Appln. Laid-Open Publn. No. 62-139709. A typical example of a known active suspension system comprises the following essential components. An individual suspension is provided for each wheel for supporting the vehicle by fluid pressure. Charging and discharging fluid into or out of each suspension are controlled independently by operations of a respective control valve. The operation of each control valve is controlled by opening and closing control signals generated by a controller responsive to information such as vertical acceleration of the vehicle mass above a suspension unit or a vertical displacement of the suspension. And the controller calculates a command quantity of charging and discharging the fluid for each suspension unit. Thus charging and discharging the fluid into and out of each suspension are controlled.
Applicant has previously developed an active suspension system described hereinafter and has filed a U.S. patent application therefor. In the active suspension system as described above, sensors are used to detect accelerations in the longitudinal and lateral directions of the vehicle. The above described controller preestimates variations of a vehicle attitude (pitching and rolling) accompanying the acceleration, the deceleration and a turn of the vehicle in response to the information from the sensors. Then, the controller computes the command quantity of charging and discharging the fluid for maintaining the vehicle attitude in a desirable state, thus generating and transmitting the signals for opening and closing the control valves.
The active suspension system described above carries out a feedback control and a predictor control. That is, in the feedback control, the system carries out a control such that a desired vehicle attitude is maintained in response to variations of a vehicle state such as extension and contraction of each suspension. In the predictor control, longitudinal and lateral accelerations are detected, and the vehicle attitude is controlled by predicting a quantity of load shift which will occur as a result of the accelerations.
In such an active suspension system, variations of vehicle weight do not effect the feedback control. However, in the predictor control, the amount of charging and discharging fluid into and out of each suspension becomes deficient or excessive when the loading condition of the vehicle varies so that the vehicle weight becomes larger or smaller than a reference vehicle weight which is the weight of a vehicle in a predetermined condition (for example, the vehicle is in a straight running condition with only one passenger, the driver without any other load). However, according to the predictor control stated above, it is impossible to maintain the vehicle attitude in a desired state under a transitional vehicle condition such as acceleration, deceleration or turning thereof.